Process for producing perfluorocarbons and use thereof

ABSTRACT

The process for producing perfluorocarbons according to the present invention is characterized in that in the production of a perfluorocarbon by contacting an organic compound with a fluorine gas, the organic compound is contacted with the fluorine gas at a temperature of from 200 to 500° C. and the content of an oxygen gas within the reaction system is controlled to 2% by volume or less based on the gas components in the reaction starting material, whereby a perfluorocarbon reduced in the content of impurities is produced. According to the process for producing perfluorocarbons of the present invention, high-purity perfluorocarbons extremely suppressed in the production of impurities such as an oxygen-containing compound can be obtained. The perfluorocarbons obtained by the production process of the present invention contain substantially no oxygen-containing compound and, therefore, can be effectively used as an etching or cleaning gas for use in a process for producing a semiconductor device.

CROSS REFERENCES OF RELATED APPLICATION

This is a divisional of application Ser. No. 10/258,172 filed Oct. 22,2002 now U.S. Pat. No. 7,064,240, which is the National Stage ofPCT/JP02/01549 filed Feb. 21, 2002 and claims benefit of ProvisionalApplication No. 60/272,451 filed Mar. 2, 2001; the above noted priorapplications are all hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a process for producingperfluorocarbons, more specifically, the present invention relates to aprocess for producing perfluorocarbons useful for the production ofsemiconductor devices and reduced in the production of impurities suchas oxygen-containing compound.

The present invention also relates to high-purity perfluorocarbonscontaining substantially no oxygen-containing compound, aperfluorocarbon-containing gas, and uses thereof.

BACKGROUND ART

In the production process of semiconductor devices, perfluorocarbonshave been conventionally used as one of useful etching or cleaninggases.

On the other hand, to keep up with recent tendency toward higherperformance, smaller size, higher density wiring and the like ofelectrical or electronic equipment, the electrode of a circuit substrateis becoming finer and in order to form a circuit pattern with higherprecision by etching or the like, use of an extremely high-purityetching gas from which impurities are eliminated as much as possible isdemanded. When the etching gas contains an impurity even if in a verysmall amount, this may cause generation of a large width line in theformation of a fine pattern or increase of defects in the product havinga high density integrated circuit.

Also, the process of removing deposits using a cleaning gas must beperformed to let residual impurities be reduced as much as possible inthe production process of a semiconductor device after the cleaning soas to provide a high-purity and high-quality device. For this purpose, ahigh-purity cleaning gas containing substantially no impurity isdemanded.

With respect to the process for producing perfluorocarbons, variousmethods have been heretofore proposed. For example, as fortetrafluoromethane, a method of reacting chlorotrifluoromethane with HFin the presence of a catalyst (see, JP-B-62-10211 (the term “JP-B” asused herein means an “examined Japanese patent publication”)) and amethod of reacting dichlorodifluoromethane with HF in the presence of acatalyst (see, JP-B-42-3004) are known; and as for hexafluoroethane, anelectrolytic fluorination method starting from ethane and/or ethylene, apyrolysis method of thermally decomposing tetrafluoroethylene, and amethod of fluorinating acetylene, ethylene, ethane or the like using ametal fluoride are known. Furthermore, a direct fluorination method ofcontacting hydrocarbon or hydrofluorocarbon with a fluorine gas is alsoknown and examples thereof include a method of reactingtrifluoro-methane with a fluorine gas (see, GB 1,116,920), a method ofreacting tetrafluoroethane with a fluorine gas (see, Japanese Patent2,947,158), a method of reacting hexafluoropropylene with a fluorine gas(see, JP-B-62-61572), and a method of reacting carbon (C) with F₂ inBrF₃ or IF₃ (see, JP-A-58-162536). Other than these, in the case ofoctafluoropropane as a perfluorocarbon having 3 carbon atoms, a directfluorination method of reacting propane with a fluorine gas is known(see, EP 31519).

Of these various production processes, the direct fluorination methoduses a fluorine gas having an extremely high reactivity and therefore,incurs dangers of bringing about explosion, corrosion or the likebetween the substrate organic compound and the fluorine gas andfurthermore, dangers of causing side reactions such as abrupt reactionor explosion resulting from cleavage of C—C bond, polymerization,production of carbon (C), volume or the like due to generation of heat.

For example, in the case of synthesizing a perfluorocarbon by the directfluorination method of reacting a linear hydrocarbon compound with afluorine gas, the synthesis is accompanied with a very large heat ofreaction as shown below.CH₄+4F₂→CF₄+4HF  (Equation 1)(ΔH=−479 kcal/mol)C₂H₆+6F₂→C₂F₆+6HF  (Equation 2)(ΔH=−690 kcal/mol)

As such, in replacing one C—H bond by C—F bond, a heat of reaction ofabout −110 Kcal/mol is generated. In the direct fluorination method ofreacting propane (C₃H₈) with a fluorine gas, ΔH is about −880 Kcal/mol.

In the case of starting from methane (Equation 1), 4 mol of fluorine gasis necessary per 1 mol of methane and in the case of starting fromethane (Equation 2), 6 mol of fluorine gas is necessary per 1 mol ofethane. In this way, as the number of hydrogen atoms in the substrateorganic compound is larger or as the amount of fluorine used is larger,the heat of reaction becomes larger. In order to prevent the abruptgeneration of heat of reaction in the direct fluorination method, therehave been proposed, for example, a method of diluting the fluorine gaswith another inert gas (e.g., nitrogen, helium), a method of diluting asubstrate organic compound with another inert gas, a method ofdissolving a substrate organic compound in a solvent inactive tofluorine in a low concentration, a method of performing the reaction ina low temperature region or a method of designing an apparatus such asjet reactor to allow the fluorine gas to gradually come into contactwith the substrate organic compound when the reaction is performed in agas phase.

The present inventors have already found that these problems encounteredin the direct fluorination method can be solved by appropriatelycontrolling the reaction conditions in the direct fluorination methodand thereby, perfluorocarbons can be safely and economically produced inindustry with good efficiency (Japanese Patent 3,067,633).

In the case of using the thus-obtained perfluorocarbons as a cleaning oretching gas in the process for producing a semiconductor device or thelike, the perfluorocarbon must be free of various impurities as much aspossible and have a high purity as described above. For the removal ofimpurities, separation by distillation or the like is usually used.Heretofore, perfluorocarbons having a fixed purity have been produced bythe direct fluorination method where impurities are removed to a certainpurity by combining, for example, purification of starting materials,and distillation and purification of the product.

In the process of studying the above-described method for obtainingperfluorocarbons by the direct fluorination method, the presentinventors have found that some components remain as impurities even byperforming high-precision distillation or the like and these residualimpurities cannot be easily and effectively removed. By the analysis ofthese impurities, oxygen-containing compounds such as perfluorodimethylether, perfluorodimethyl peroxide and perfluoromethyl ethyl ether weredetected. These oxygen-containing compounds were very difficult toremove because these formed an azeotropic composition or azeotrope-likemixture with perfluorocarbons. If such a perfluorocarbon is used as anetching or cleaning gas for the manufacture of a semiconductor devicewhile allowing mixing of those oxygen-containing compounds in a highconcentration, the requirement for formation of a very fine pattern maynot be satisfied.

Accordingly, the present inventors have made extensive investigations toprevent the production of such oxygen-containing compounds, as a result,it has been found that these oxygen-containing compounds are originatedin the oxygen gas contained in a slight amount in the reaction startingmaterials such as fluorine gas or hydrofluorocarbon, and when the oxygengas content in the reaction starting materials is reduced to a specificamount or less while controlling the reaction conditions such asreaction temperature to fall within a certain range, the production ofthose oxygen-containing compounds can be effectively prevented. Thepresent invention has been accomplished based on this finding. To thebest knowledge of the present inventors, there has been not found aprocess for producing perfluorocarbons, which involves a technique ofpreventing the production of oxygen-containing compounds originated inthe oxygen gas in starting materials.

OBJECT OF THE INVENTION

The present invention has been made to overcome the above-describedproblems in conventional techniques and the object of the presentinvention is to provide high-purity perfluorocarbons suppressed from theproduction of impurities such as oxygen-containing compound, and providea production process therefor. The object of the present inventionincludes providing uses of the high-purity perfluorocarbons.

SUMMARY OF THE INVENTION

The process for producing perfluorocarbons according to the presentinvention is characterized in that in the production of perfluorocarbonsfrom a reaction starting material comprising an organic compound and afluorine gas, the organic compound is contacted with the fluorine gas ata temperature of from 200 to 500° C. while controlling the content of anoxygen gas within the reaction system to 2% by volume or less based onthe gas components in the reaction starting material to produce aperfluorocarbon reduced in the content of impurities.

The organic compound and the fluorine gas are preferably contacted inthe presence of a diluting gas.

The organic compound is preferably an aliphatic saturated compoundhaving 6 or less carbon atoms and/or an aliphatic unsaturated compoundhaving 6 or less carbon atoms.

The organic compound is more preferably an aliphatic saturated compoundhaving 6 or less carbon atoms, still more preferably ahydrofluorocarbon, and particularly preferably at least one memberselected from the group consisting of fluoromethane, difluoromethane,trifluoromethane, trifluoroethane, tetrafluoroethane, pentafluoroethane,pentafluoropropane, hexafluoropropane and heptafluoropropane.

In the process for producing perfluorocarbons, which is a process forproducing a perfluorocarbon by contacting the organic compound with thefluorine gas in a gas phase using no catalyst in the presence of adiluting gas, the oxygen gas contained in the fluorine gas before thecontact between the organic compound and the fluorine gas is preferablyin an amount of 1% by volume or less based on said fluorine gas. In thiscase, the organic compound is preferably a hydrofluorocarbon having 4 orless carbon atoms, more preferably at least one member selected from thegroup consisting of difluoromethane, trifluoromethane, trifluoroethane,tetrafluoroethane, pentafluoroethane, hexafluoropropane andheptafluoropropane.

The amount of fluorine gas introduced into the reaction system ispreferably 9% by volume or less based on the total amount of gascomponents within the reaction system.

The diluting gas is at least one member selected from the groupconsisting of tetrafluoromethane, hexafluoroethane, octafluoropropaneand hydrogen fluoride.

The diluting gas preferably contains hydrogen fluoride and the contentof the hydrogen fluoride is preferably 50% by volume or more based onthe entire amount of diluting gas.

The impurity may be an oxygen-containing compound.

The process for producing perfluorocarbons may further comprise a stepof adsorbing and thereby removing the oxygen-containing compound.

The oxygen-containing compound can be adsorbed and thereby removed byactivated carbon.

The perfluorocarbon is preferably at least one member selected from thegroup consisting of tetrafluoromethane, hexafluoroethane andoctafluoropropane.

The total amount of oxygen-containing compounds contained in theperfluorocarbons is preferably 5 ppm by volume or less, more preferably2 ppm by volume or less.

The perfluorocarbon-containing gas of the present invention contains theabove-described perfluorocarbons.

The etching gas of the present invention is characterized by comprisingthe above-described perfluorocarbon-containing gas, and theperfluorocarbon is preferably tetrafluoromethane.

The cleaning gas of the present invention is characterized by comprisingthe above-described perfluorocarbon-containing gas, and theperfluorocarbon is preferably hexafluoroethane or octafluoropropane.

DETAILED DESCRIPTION OF THE INVENTION

The process for producing perfluorocarbons according to the presentinvention and uses thereof are described in detail below.

[Process for Producing Perfluorocarbons]

The process for producing perfluorocarbons according to the presentinvention is a process for producing a perfluorocarbon by contacting anorganic compound with a fluorine gas (F₂), where the content of oxygengas contained in the reaction system is controlled to a fixed amount orless. The process for producing perfluorocarbons according to thepresent invention may be performed, if desired, in the presence of adiluting gas.

(Organic Compound)

The organic compound which can be used in the present invention is notparticularly limited and known organic compounds used in the productionof perfluorocarbon may be used.

Examples of the organic compound include aliphatic saturated compoundshaving 6 or less carbon atoms and aliphatic unsaturated compounds having6 or less carbon atoms. At least one organic compound selected fromthese compounds is preferably used. Among these compounds, the organiccompound for use in the present invention is more preferably at leastone aliphatic saturated compound having 6 or less carbon atoms.

Examples of the aliphatic saturated compound having 6 or less carbonatoms include hydrofluorocarbons such as fluoromethane, difluoromethane,tifluoromethane, trifluoroethane, tetrafluoroethane, pentafluoroethane,pentafluoropropane, hexafluoropropane and heptafluoropropane. Thealiphatic saturated compound is preferably at least onehydrofluorocarbon selected from these compounds.

In the case where the organic compound and the fluorine gas arecontacted in a gas phase using no catalyst in the presence of a dilutinggas, the organic compound is preferably a hydrofluorocarbon having 4 orless carbon atoms.

Examples of this hydrofluorocarbon include difluoromethane,trifluoromethane, tetrafluoroethane, pentafluoroethane,hexafluoropropane and heptafluoropropane. The organic compound ispreferably at least one hydrofluorocarbon selected from these compounds.

Perfluorocarbons such as tetrafluoromethane, hexafluoroethane andoctafluoropropane can be preferably obtained starting from theabove-described hydrofluorocarbons.

(Fluorine Gas)

The fluorine gas for use in the present invention can be produced by aknown method such as electrolysis of hydrogen fluoride. Also, acommercially available fluorine gas can be used.

(Diluting Gas)

Examples of the diluting gas which can be used in the present inventioninclude tetrafluoromethane, hexafluoroethane, octafluoropropane andhydrogen fluoride.

These diluting gases may be used individually or in combination of twoor more thereof. Among these diluting gases, hydrogen fluoride ispreferred. In the case of using hydrogen fluoride with another dilutinggas, the gas used in combination is preferably a gas rich in hydrogenfluoride. More specifically, hydrogen fluoride is preferably in anamount of 50% by volume or more, more preferably 60% by volume or more,based on the entire diluting gas.

(Process for Producing Perfluorocarbons)

In the case where tetrafluoroethane (CF₃CH₂F) or trifluoromethane (CHF₃)is used as the starting material organic compound and contacted with afluorine gas to produce tetrafluoromehtane (CF₄) or hexafluoroethane(CF₃CF₃), if an oxygen gas is present in the starting materials (organiccompound and fluorine gas), oxygen-containing compounds such as CF₃OCF₃,CF₃OOCF₃ and CF₃CF₂OCF₃ are produced. These oxygen-containing compoundsform an azeotropic composition or azeotrope-like mixture with theobjective product and it is very difficult to separate theseoxygen-containing compounds from the objective product by a known methodsuch as distillation.

In the process for producing perfluorocarbons according to the presentinvention, the organic compound and the fluorine gas (F₂) are contactedto produce a perfluorocarbon and at this time, the content of an oxygengas contained in the reaction gas is suitably controlled to 2% by volumeor less, preferably 1% by volume or less, more preferably 0.5% by volumeor less, based on the gas components within the reaction system.

The term “reaction system” as used herein means a reaction solution orgas phase atmosphere containing a reaction starting material comprisingan organic compound and a fluorine gas, and if desired, furthercontaining a diluting gas, within a reactor where the organic compoundand the fluorine gas are actually contacted. The reaction system is theabove-described reaction solution or gas phase atmosphere within areactor between the charging of starting materials and the completion ofreaction but excludes the reaction solution or gas phase atmosphere inthe steps after the completion of reaction, such as extraction andpurification.

The reaction between the organic compound and the fluorine gas may alsobe performed in a gas phase using no catalyst in the presence of adiluting gas. In this case, the content of an oxygen gas containing inthe starting material fluorine gas is preferably controlled to 1% byvolume or less, more preferably 0.6% by volume or less, still morepreferably 0.4% by volume or less, based on the fluorine gas.

When the oxygen gas content in the reaction system is less than 2% byvolume, high-purity perfluorocarbons substantially free of theabove-described oxygen-containing compounds can be produced.Furthermore, when the oxygen gas content in the fluorine gas is reducedto 1% by volume or less, more high-purity perfluorocarbons substantiallyfree of oxygen-containing compounds can be produced.

If the amount of oxygen gas contained within the reaction system exceeds2% by volume, a significant amount of oxygen-containing compounds areproduced.

In order to control the oxygen gas content within the reaction systemand in the fluorine gas to fall within the above-described range, it isnecessary to perform the reaction in a closed system not to allow anoxygen gas from mingling into the reactor from the outside and to removean oxygen gas from the reaction starting materials such as organiccompound, fluorine gas and diluting gas. In the case where a part of thegas after the reaction is circulated and used as a diluting gas, thepossibility of an oxygen gas mingling from the outside advantageouslydecreases. The oxygen gas contained in the organic compound, fluorinegas and diluting gas can be removed by a known method. This may beattained by separating impurities in the starting materials, forexample, through distillation of reaction starting materials oradsorption using an adsorbent such as activated carbon.

In the case of contacting the organic compound with the fluorine gas inthe presence of a diluting gas, either one or both of the reactionsubstrate (organic compound) and the fluorine gas may be diluted with adiluting gas before charging the organic compound or fluorine gas intothe reactor.

Of these, the fluorine gas is preferably diluted with a diluting gasbefore charging.

In this case, the amount of fluorine gas introduced into the reactionsystem is preferably 9% by volume or less, more preferably 8% by volumeor less, based on the total amount of gas components within the reactionsystem. In the case of continuously performing the reaction, the amountof fluorine gas introduced into the reaction system is preferablycontrolled to always fall within the above-described range.Incidentally, the gas components within the reaction system meansfluorine gas, organic compound and diluting gas within the reactionsystem.

As described above, the direct fluorination method of producing aperfluorocarbon using a fluorine gas uses a fluorine gas extremely richin reactivity and therefore, when the substrate organic compound,particularly hydrogen-containing compound, in a high concentration isexposed to fluorine, this may incur combustion or explosion. When thefluorine gas concentration is 9% by volume or less at the inlet of thereactor, the mixed gas concentration can be outside the range ofexplosion and the reaction between the fluorine gas and the organiccompound can be performed safely in industry.

In the present invention, the reaction temperature on contacting theorganic compound with the fluorine gas is suitably from 200 to 500° C.,preferably from 300 to 450° C.

When the reaction temperature is within this range, the production ofoxygen-containing compounds produced as impurities originated in theoxygen gas can be remarkably reduced. If the reaction temperatureexceeds 500° C., even if the oxygen gas (concentration) within thereaction system is controlled to 2% by volume or less or the oxygen gasconcentration in the fluorine gas is controlled to 1% by volume or less,a significant amount of oxygen-containing compounds are sometimesproduced. Accordingly, in the present invention, the reactiontemperature preferably controlled within the above-described range. Thiscontrol of the reaction temperature is preferably performed without failusing, for example, thermocouple such that the reaction temperature doesnot exceed 500° C. not only in the reaction zone within the reactor butalso in the portions where the organic compound or the fluorine gas ispresent.

According to the present invention, the mingling of oxygen-containingcompounds is outstandingly prevented, however, in the case where theobtained crude perfluorocarbon contains a slight amount of oxygencontaining compounds or impurities such as nitrogen, carbon monoxide andcarbon dioxide, a step of adsorbing and thereby removing these compoundsor impurities using activated carbon or the like is preferably provided.

The activated carbon used here may be a known activated carbon and amongknown activated carbons, coconut shell carbon can be preferably used.The separating and thereby removing operation may be performed in eitherliquid phase or gas phase but is preferably performed in gas phase.

Perfluorocarbons

The total amount of oxygen-containing compounds contained in thethus-obtained perfluorocarbons can be reduced to, in the case of crudeperfluorocarbons before purification, preferably 5 ppm by volume orless, more preferably 2 ppm by volume or less, still more preferably 1ppm by volume or less. Thus, according to the production process of thepresent invention, high-purity perfluorocarbons remarkably reduced inthe content of oxygen-containing compounds can be obtained withoutpassing through a purification step such as distillation or adsorption.

Also, by passing the purification step, the total content ofoxygen-containing compounds can be reduced to preferably 1 ppm by volumeor less, more preferably 0.5 ppm by volume or less, still morepreferably 0.4 ppm by volume or less. Thus, according to the productionprocess of the present invention, high-purity perfluorocarbons morereduced in the content of oxygen-containing compounds can be very easilyand simply obtained.

The oxygen-containing compounds can be detected (analyzed) usinganalysis methods such as TCD, FID and DID methods of gas chromatography(GC), and gas chromatography-mass spectrometer (GC-MS).

Perfluorocarbons-Containing Gas and Use Thereof

The perfluorocarbons obtained by the production process of the presentinvention are satisfactorily reduced in impurities such asoxygen-containing compound and therefore, can be used over a wide range.For example, the compound which is a gas at ordinary temperature can beused as an etching gas at the etching step in the production process ofa semiconductor device and the compound which is a liquid at ordinarytemperature can be used as a cooling solvent or the like.

More specifically, in the production process of a semiconductor devicesuch as LSI and TFT, the compound can be suitably used as an etching gasfor forming a circuit pattern after forming a thin or thick film using aCVD method, a sputtering method or a vapor deposition method.

The compound can also be used as a cleaning gas at the cleaning step inthe production process of a semiconductor device.

In the case of using the perfluorocarbons of the present invention as anetching gas, tetrafluoromethane is preferred and in the case of using asa cleaning gas, hexafluoroethane or octafluoropropane is preferred.

More specifically, in an apparatus for forming a thin or thick film,cleaning is performed to remove unnecessary deposits accumulated on theinner wall of the apparatus, a jig and the like, because unnecessarydeposits produced cause generation of particles and must be removed onoccasions for producing a film of good quality. The perfluorocarbonsaccording to the present invention can be suitably used as a cleaninggas therefor.

The gas containing a high-purity perfluorocarbon according to thepresent invention is a gas containing a perfluorocarbon which is a gasat ordinary temperature. The gas may contain the perfluorocarbon aloneor may appropriately contain other gas. Examples of the other gasinclude inert gases such as He, Ne and Ar. The amount of the other gasblended is not particularly limited. For example, in the case of usingthe high-purity perfluorocarbons according to the present invention asan etching or cleaning gas, the amount of the other gas blended variesdepending on the kind, thickness and the like of the compound to beetched and can be determined according to the amount and thickness ofthe deposit to be cleaned.

Effects of the Invention

According to the process for producing perfluorocarbons of the presentinvention, the oxygen gas content in the reaction system is reduced to aspecific amount or less, so that high-purity perfluorocarbons extremelysuppressed in the production of impurities such as oxygen-containingcompound can be obtained. The perfluorocarbons obtained by theproduction process of the present invention contain substantially nooxygen-containing compound and therefore, can be effectively used as anetching or cleaning gas for use in the process for producing asemiconductor device.

EXAMPLES

The present invention is described in greater detail below by referringto Examples, however, the present invention should not be construed asbeing limited to these Examples.

Preparation Example 1

A fluorination reaction was performed by contacting chloroform (CHCl₃)and hydrogen fluoride (HF) in gas phase in the presence of afluorination catalyst. The reaction product was purified by a knowndistillation method to obtain crude trifluoromethane (CHF₃). Theobtained crude trifluoromethane was analyzed by gas chromatography andfound to have the following composition.

CHF₃ 97.2667 0.1126 1.2081 Nitrogen gas 1.4126

unit: % by volume

Incidentally, other impurities such as CHClF₂ and CClF₃ were contained.

The results obtained are shown in Table 1.

Preparation Example 2

A distillation operation was repeatedly applied to the crudetrifluoromethane obtained in Preparation Example 1. The obtainedtrifluoromethane was analyzed by gas chromatography and found to havethe following composition.

CHF₃ 99.8288 Other organic impurities 0.0208 Oxygen gas 0.0720 Nitrogengas 0.0784

The results obtained are shown in Table 1.

Preparation Example 3

A fluorination reaction was performed by contacting trichloroethyleneand hydrogen fluoride in gas phase in the presence of a fluorinationcatalyst. The reaction product was purified by a known distillationmethod to obtain crude tetrafluoroethane (CF₃CH₂F). The obtained crudetetrafluoroethane was analyzed by gas chromatography and found to havethe following composition.

CF₃CH₂F 97.0359 Other organic impurities 0.5124 Oxygen gas 1.3314Nitrogen gas 1.1203

unit: % by volume

Incidentally, other impurities such as CF₃CH₃, CF₃CHF₂ and CHF₂CHF₂ werecontained.

The results obtained are shown in Table 1.

Preparation Example 4

A distillation operation was repeatedly applied to the crudetetrafluoroethane obtained in Preparation Example 3. The obtainedtetrafluoroethane was analyzed by gas chromatography and found to havethe following composition.

CF₃CH₂F 99.9018 Other organic impurities 0.0088 Oxygen gas 0.0402Nitrogen gas 0.0492

unit: % by volume

The results obtained are shown in Table 1.

Preparation Example 5

A fluorine gas was obtained by the electrolysis of hydrogen fluoride.The obtained fluorine gas was sampled using an SUS cylinder (having aninner surface subjected to a passivation treatment) and after removingfluorine, analyzed by gas chromatography. As a result, the oxygen gas inthe fluorine gas was found to have the following concentration value.

Oxygen gas 1.3825 unit: % by volume

The main component in the remaining was fluorine and nitrogen gas,hydrogen fluoride and the like were contained.

Preparation Example 6

The starting material obtained in Preparation Example 5 was furthersubjected to a purification operation (e.g., cooling) and analyzed inthe same manner as in Preparation Example 5. As a result, the oxygen gasin the fluorine gas was found to have the following concentration value.

Oxygen gas 0.3020 unit: % by volume

The results obtained are shown in Table 1.

Example 1

An Inconel 600-made reactor having an inner diameter of 20.6 mmØ and alength of 500 mm (electric heater heating system, the reactor wassubjected to a passivation treatment with a fluorine gas at atemperature of 600° C.) was heated to a temperature of 420° C. whilefeeding a nitrogen gas at 30 NL/hr. Thereafter, hydrogen fluoride wasfed at 50 NL/hr and furthermore, while passing a diluting gas comprisingthose nitrogen and hydrogen fluoride was into one side of branched gasflow, the trifluoromethane obtained in Preparation Example 2 was flowedat 3.6 NL/hr.

Thereafter, while passing the same diluting gas comprising nitrogen andhydrogen fluoride into another side of branched gas flow, the fluorinegas prepared in Preparation Example 6 was fed at a flow rate of 3.9NL/hr, thereby performing the reaction.

After 3 hours, the reaction outlet gas was treated with an aqueouspotassium hydroxide solution and an aqueous potassium iodide solution toremove hydrogen fluoride and unreacted fluorine gas and then analyzed bygas chromatography. The analysis results of the organic compositionexcept for oxygen and nitrogen portions are shown below.

CF₄ 98.7992 CF₃CF₃ 0.4808 Others 0.7200

unit: % by volume

“Others” were C₃F₈, CClF₃ and the like and the total amount of CF₃OCF₃and CF₃OOCF₃ as oxygen-containing compounds was 2 ppm by volume or less.

Thereafter, the reaction outlet gas was treated with an aqueouspotassium hydroxide solution and an aqueous potassium iodide solution toremove hydrogen fluoride and unreacted fluorine gas and the resultinggas was passed through a dehydrating agent, collected in an SUS cylinderunder cooling, and then distillation-purified by a known distillationoperation to obtain tetrafluoromehtane. The organic composition thereofwas analyzed by gas chromatography and gas chromatography-massspectrometer. The results are shown below.

CF₃OCF₃ <0.2 ppm by volume CF₃OOCF₃ <0.2 ppm by volume Others <2.0 ppmby volume CF₄ >99.9997% by volume

The total amount of oxygen-containing compounds was 0.5 ppm by volume orless.

The results obtained are shown in Table 2.

Example 2

While feeding a nitrogen gas at 30 NL/hr, the same reactor as in Example1 was heated to a temperature of 370° C. Then, hydrogen fluoride was fedat 50 NL/hr and furthermore, while passing a diluting gas comprisingthose nitrogen and hydrogen fluoride into one side of branched gas flow,a gas mainly comprising the tetrafluoroethane prepared in PreparationExample 4 was flowed at 1.8 NL/hr. Thereafter, while passing the samediluting gas into another side of branched gas flow, the fluorine gasprepared in Preparation Example 6 was fed at 3.9 NL/hr, therebyperforming the reaction.

After 3 hours, the reaction outlet gas was treated with an aqueouspotassium hydroxide solution and an aqueous potassium iodide solution toremove hydrogen fluoride and unreacted fluorine gas and then analyzed bygas chromatography. The analysis results of the organic compositionexcept for oxygen and nitrogen portions are shown below.

CF₃CF₃ 98.4875 CF₃CHF₂ 0.0025 CF₄ 0.7280 Others 0.7820

unit: % by volume

“Others” were mainly C₃F₈ and the total amount of CF₃CF₂OCF₃, CF₃OCF₃and CF₃OOCF₃ as oxygen-containing compounds was 1 ppm by volume or less.

Thereafter, the reaction outlet gas was treated with an aqueouspotassium hydroxide solution and an aqueous potassium iodide solution toremove hydrogen fluoride and unreacted fluorine gas and the resultinggas was passed through a dehydrating agent, collected in an SUS cylinderunder cooling, and then distillation-purified by a known distillationoperation to obtain hexafluoroethane (CF₃CF₃). The organic compositionthereof was analyzed by gas chromatography and gas chromatography-massspectrometer. The results are shown below.

CF₃OCF₃ <0.2 ppm by volume CF₃OOCF₃ <0.1 ppm by volume CF₃CF₂OCF₃ <0.1ppm by volume Others <2.0 ppm by volume CF₃CF₃ >99.9997% by volume

The results obtained are shown in Table 2.

Comparative Example 1

While feeding a nitrogen gas at 30 NL/hr, the same reactor as in Example1 was heated to a temperature of 450° C. Then, hydrogen fluoride was fedat 50 NL/hr and furthermore, while passing a diluting gas comprisingthose nitrogen and hydrogen fluoride into one side of branched gas flow,a gas mainly comprising the tetrafluoromethane prepared in PreparationExample 1 was flowed at 3.6 NL/hr. Thereafter, while passing the samediluting gas into another side of branched gas flow, the fluorine gasprepared in Preparation Example 5 was fed at 3.9 NL/hr, therebyperforming the reaction.

After 3 hours, the reaction outlet gas was treated with an aqueouspotassium hydroxide solution and an aqueous potassium iodide solution toremove hydrogen fluoride and unreacted fluorine gas and then analyzed bygas chromatography. The analysis results of the organic compositionexcept for oxygen and nitrogen are shown below.

CF₄ 98.3618 CF₃CF₃ 0.4988 Others 1.1123 CF₃OCF₃ 0.0146 (146 ppm)CF₃OOCF₃ 0.0125 (125 ppm)unit: % by volume

With a total oxygen content (concentration) of 2% or more in thestarting material, a significant amount of oxygen-containing compoundsas impurities were produced.

The results obtained are shown in Table 2.

Comparative Example 2

While feeding a nitrogen gas at 30 NL/hr, the same reactor as in Example1 was heated to a temperature of 430° C. Then, hydrogen fluoride was fedat 50 NL/hr and furthermore, while passing a diluting gas comprisingthose nitrogen and hydrogen fluoride into one side of branched gas flow,a gas mainly comprising the tetrafluoroethane prepared in PreparationExample 3 was flowed at 1.8 NL/hr. Thereafter, while passing the samediluting gas into another side of branched gas flow, the fluorine gasprepared in Preparation Example 5 was fed at 3.9 NL/hr, therebyperforming the reaction.

After 3 hours, the reaction outlet gas was treated with an aqueouspotassium hydroxide solution and an aqueous potassium iodide solution toremove hydrogen fluoride and unreacted fluorine gas and then analyzed bygas chromatography. The analysis results of the organic compositionexcept for oxygen and nitrogen are shown below.

CF₃CF₃ 97.1841 CF₄ 0.8920 Others 1.8920 CF₃OCF₃ 0.0133 (133 ppm)CF₃OOCF₃ 0.0088 (88 ppm)  CF₃CF₂OCF₃ 0.0098 (98 ppm) 

unit: % by volume

Thereafter, the reaction outlet gas was treated with an aqueouspotassium hydroxide solution and an aqueous potassium iodide solution toremove hydrogen fluoride and unreacted fluorine gas and the resultinggas was passed through a dehydrating agent, collected in an SUS cylinderunder cooling, and then distillation-purified by a known distillationoperation to obtain hexafluoroethane. The organic composition thereofwas analyzed by gas chromatography and gas chromatography-massspectrometer. The results are shown below.

CF₃CF₃ >99.9866% by volume CF₄ <0.4 ppm by volume Others <2.0 ppm byvolume CF₃OCF₃ 128 ppm by volume CF₃OOCF₃ 1 ppm by volume CF₃CF₂OCF₃ 1ppm by volume

CF₃OCF₃ forms an azeotropic mixture with the objective hexafluoroethaneand the separation thereof is apparently difficult.

The results obtained are shown in Table 2.

Comparative Example 3

A gas obtained using the same reactor as in Example 1 thoroughly in thesame conditions by the same operations except for changing the reactiontemperature to 520° C. was analyzed by gas chromatography. The analysisresults of the organic composition except for oxygen and nitrogen areshown below.

CF₄ 95.3548 CF₃CF₃ 1.6872 Others 2.8945 CF₃OCF₃ 0.0367 CF₃OOCF₃ 0.0268

unit: % by volume

The results obtained are shown in Table 2.

TABLE 1 Oxygen Gas Content Product (vol %) Preparation Example 1trifluoromethane 1.2081 Preparation Example 2 trifluoromethane 0.0720Preparation Example 3 tetrafluoroethane 1.3314 Preparation Example 4tetrafluoroethane 0.0402 Preparation Example 5 F₂ 1.3825 PreparationExample 6 F₂ 0.3020

TABLE 2 Concentration of Oxygen- Concentration of Oxygen- TotalConcentration of Oxygen Containing Compounds in Containing Compoundsafter Reaction Starting Reaction Gas in Reaction Starting MaterialPerfluorocarbons (ppm by Purification Material Temperature (° C.) (% byvolume) volume) (ppm by volume) Example 1 Preparation 420 0.3740 2 orless 0.5 or less Example 2 Preparation Example 6 Example 2 Preparation370 0.3422 1 or less 0.4 or less Example 4 Preparation Example 6Comparative Preparation 450 2.5906 271 Example 1 Example 1 PreparationExample 5 Comparative Preparation 430 2.7139 319 130 Example 2 Example 3Preparation Example 5 Comparative Preparation 520 0.3740 635 Example 3Example 2 Preparation Example 6

1. Perfluorocarbons having an oxygen-containing compound content of 5ppm by volume or less.
 2. The perfluorocarbons as claimed in claim 1,wherein the oxygen-containing compound content is 2 ppm by volume orless.
 3. A gas containing the perfluorocarbons described in claim
 1. 4.An etching gas comprising the gas described in claim
 3. 5. The etchinggas as claimed in claim 4, wherein said perfluorocarbon istetrafluoromethane.
 6. A cleaning gas comprising the gas described inclaim
 3. 7. The cleaning gas as claimed in claim 6, wherein saidperfluorocarbon is hexafluoroethane.
 8. The cleaning gas as claimed inclaim 6, wherein said perfluorocarbon is octafluoropropane. 9.Perfluorocarbons having an oxygen-containing compound content of 5 ppmby volume or less, which are obtainable by a process comprising, in theproduction of a perfluorocarbon from a reaction starting materialcomprising an organic compound and a fluorine gas, contacting an organiccompound with a fluorine gas at a temperature of from 200 to 500° C.while controlling the content of an oxygen gas within the reactionsystem to 2% by volume or less based on the gas components in thereaction starting material.